Optical Signal Aiming for Heliostats

ABSTRACT

Methods and systems for managing heliostat aiming toward a target are described. Solar rays incident on a reflective surface of a heliostat mirror are reflected toward the target. One or more optical signalers are arranged at positions about the target. An optical signal received from one of the one or more optical signalers is detected. An error in an orientation of the reflective surface is determined based on the optical signal.

TECHNICAL FIELD

This specification relates to aiming heliostats toward solar energy receivers.

BACKGROUND

Heliostats can be used to collect radiation from the Sun. Specifically, a heliostat can include one or more mirrors to direct solar rays toward a receiver mounted on a receiver tower. Some types of heliostats are capable of moving their mirror or mirrors as the Sun moves across the sky, both throughout the day and over the course of the year, in order to more efficiently direct solar rays to the receiver. Solar rays that are directed to the receiver can then be used to generate solar power. A field of heliostats can be placed surrounding one or more receivers to increase the quantity of radiation collected and optimize the amount of solar power that is generated.

The solar energy can be converted to electricity by the receiver or a generator that is coupled to the receiver. Typically, a working fluid that circulates within a receiver is heated by solar energy incident on the receiver. The heated working fluid can then be used to power a turbine and generator to produce electricity.

SUMMARY

In general, in one aspect, a method for managing heliostat aiming toward a target is described. Solar rays incident on a reflective surface of a heliostat mirror are reflected toward the target. One or more optical signalers are arranged at positions about the target. An optical signal received from one of the one or more optical signalers is detected. An error in an orientation of the reflective surface is determined based on the optical signal.

These and other embodiments can each optionally include one or more of the following features. The orientation of the reflective surface can be adjusted in response to determining the error. Adjusting the orientation can include adjusting at least one of the azimuth or elevation of the reflective surface or adjusting the orientation can include adjusting the orientation along an axis other than along an azimuthal or elevational axis of the reflective surface. The one or more optical signalers can be multiple optical signalers and determining an error in an orientation can include determining which optical signaler of the multiple optical signalers transmitted the optical signal.

The one or more optical signalers can include multiple retroreflectors positioned about the target. The optical signal can be received from a particular one of the retroreflectors. In some implementations, determining the error in the orientation includes determining a frequency of a change in light that forms the optical signal and, based on the frequency, determining that the optical signal was received from the particular one of the multiple retroreflectors. In some implementations, determining the error in the orientation includes determining a color of light that forms the optical signal and, based on the color, determining that the optical signal was received from the particular one of the multiple retroreflectors. In some implementations, determining the error in the orientation includes determining a polarization of light forming the optical signal and, based on the polarization, determining that the optical signal was received from the particular one of the multiple retroreflectors. In some implementations, determining the error in the orientation includes determining a phasing of light forming the optical signal and, based on the phasing, determining that the optical signal was received from the particular one of the multiple retroreflectors. Based on determining that the optical signal was received from the particular retroreflector, the error in orientation can be determined.

Determining an error in an orientation of the reflective surface based on the optical signal can include determining that the optical signal exceeds a threshold signal strength. Two optical signals can be received from two optical signalers. Determining an error in an orientation of the reflective surface based on the two optical signals can include determining that a difference in signal strength between the two optical signals exceeds a threshold difference.

The target can be a receiver configured to receive solar rays. The target can be a location that is a distance away from a receiver that is configured to receive solar rays, such that solar rays from the heliostat mirror are not reflected to the receiver.

In general, in another aspect, a system is described. The system includes a receiver assembly that includes a receiver tower, a receiver mounted on the receiver tower, an aperture included in the receiver and one or more signalers positioned at one or more distances from the aperture. The receiver tower is a support structure configured to support the receiver. The aperture is configured to receive solar rays reflected from multiple heliostats. The one or more signalers are configured to receive solar rays reflected from at least one of the heliostats and, in response, to transmit an optical signal toward the one heliostat.

These and other embodiments can each optionally include one or more of the following features. The one or more signalers can be one or more retroreflectors mounted at one or more positions proximate to a circumference of the aperture. The one or more signalers can be multiple signalers and each signaler can be configured to transmit an optical signal that is different than an optical signal transmitted by each of the other signalers. Each signaler can be a retroreflector that is configured to transmit an optical signal that has a different frequency of change in signal than optical signals transmitted by the other signalers. Each signaler can be a retroreflector that is configured to transmit an optical signal that has a different color than optical signals transmitted by the other signalers. Each signaler can be a retroreflector that is configured to transmit an optical signal that has a different polarization than optical signals transmitted by the other signalers. Each signaler can be a retroreflector that is configured to transmit an optical signal that has a different phasing than optical signals transmitted by the other signalers.

The system can further include multiple heliostats. Each heliostat can include a reflective surface that is configured to reflect solar rays incident on the surface toward the receiver. Each heliostat can further include a sensor that is configured to receive optical signals from the one or more signalers. Each heliostat can include an actuator configured to adjust an orientation of the reflective surface and a controller. The controller is configured to determine errors in orientation of the reflective surface based on the optical signals received by the sensor and is further configured to provide signals to the actuator to adjust the orientation of the reflective surface in response to the determined errors.

The one or more signalers can be multiple signalers, and each signaler can be configured to transmit an optical signal that is different than an optical signal transmitted by the other signalers. The controller can be further configured to determine which particular signaler from the multiple signalers transmitted a particular optical signal based on the difference in optical signals transmitted by the multiple signalers. The controller can be further configured to determine that the received optical signals exceed a threshold signal strength. The controller can be further configured to determine whether a difference between signal strengths of two optical signals received from two optical signalers exceeds a threshold difference and determine an error in orientation based on the determination.

Particular embodiments of the subject matter described in this specification can be implemented so as to realize one or more of the following advantages. The amount of time a heliostat is reflecting sunlight off-target, that is, not toward a desired location at a receiver, can be minimized, thereby increasing the solar energy received by the receiver. An error detection system to detect that the heliostat is off-target can be formed using relatively inexpensive components. The optical signalers at the receiver, which in some implementations are retroreflectors, can be relatively inexpensive although durable and reliable devices that can endure the hot temperatures experienced at the receiver. For example, an optical signaler can be a retroreflector made with quartz, which can easily withstand high temperatures.

The details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a solar energy system including a receiver in a field of heliostats.

FIGS. 2A and 2B show schematic representations of the front surface of the receiver shown in FIG. 1.

FIG. 3 is a schematic representation of frequency and phase modulated optical signals and an error detection algorithm

FIG. 4 is a schematic representation of phased optical signals.

FIG. 5 shows a block diagram representing an example system that includes a receiver and a heliostat.

FIG. 6 is a flowchart showing an example process for maintaining a heliostat on-target.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

Methods and systems for managing heliostat aiming toward a target, for example a solar energy receiver, are described that can minimize the time a heliostat is aiming off-target. Solar rays incident on a reflective surface of the heliostat's mirror are directed toward a target, which generally is a receiver but can be a location other than the receiver. The receiver may be in a field with several, perhaps hundreds, of heliostat mirrors directing solar rays toward the receiver. Misalignment of a heliostat mirror can cause the reflected solar rays to miss, or at least partially miss, their target. In the methods and systems described, an optical signal is sent from an optical signaler at or some distance from the target (in one example, from at or near the receiver) to a heliostat that is directing solar rays off-target. The optical signaler can be positioned near the edge of a target location for fine resolution error detection and further away from the target location for coarser resolution error detection, or both. The heliostat can detect the optical signal and an error in orientation of the heliostat's reflective surface (i.e., mirror) can be determined based on the optical signal. The heliostat's mirror can then be repositioned accordingly to correct the error and resume reflecting solar rays on-target.

The optical signaler is a device that, in response to receiving light incident on a surface of the optical signaler from a source, transmits an optical signal back to the source with a minimum scattering of light. In some implementations, the optical signalers are retroreflectors. A retroreflector reflects light incident on a surface of the retroreflector, from a source, back to the source with a minimum scattering of light. The light reflected back to the source by the retroflector is in a parallel but opposite direction to the light that was incident on the retroreflector. The light reflected back by the retroreflector is the optical signal from the optical signaler to the heliostat. The retroreflector can include glass or quartz or other temperature-tolerant materials. An example retroreflector, without limitation and for illustrative purposes, is the trihedral prism retroreflector available from Thorlabs of Newton, N.J., which reflects a light beam parallel to an incident beam within 3 arc-minutes. Other illustrative examples of retroreflectors include those made with mirror flat surfaces and optics with spherical mirrors. The retroreflector can operate in the visible and/or infra-red part of the spectrum.

For illustrative purposes, the methods and systems described below in the context of a receiver being the target of solar rays reflected from heliostats. However, as discussed above, it should be understood that the methods and systems can be used in other implementations, such as when the target is not a receiver. For example, a target location may be selected that is a distance away from a receiver, such that solar rays reflected from one or more heliostats are not incident on the receiver, e.g., during periods of high solar intensity.

FIG. 1 is a schematic representation of a solar energy system 100 including a solar energy receiver 104 in a field of heliostats 120. The receiver 104 is mounted on a receiver tower 106 which is secured to a terrestrial surface 108. The heliostats 120 are each able to vary the direction in which their one or more reflective surfaces are pointing. Arrows 104, 144 and 146 represent some of the reflected sunlight. The receiver 104 is configured to receive solar rays reflected by the heliostats 120. The heliostats' 120 pitches and angles can be varied throughout the day to track the Sun as it appears to move across the daytime sky in order to maintain their reflective relationship with the receiver 104.

A control system can be configured to control the positioning of one or more reflective surfaces included on each of the heliostats 120 based on positions of the heliostats relative to the Sun 102. In some implementations, the control system may provide signals to a drive system to substantially control the pitch and angle of the heliostat mirrors to control the direction in which their light is reflected. In some implementations, the control system is implemented as a controller, e.g., controller 136, at each of the individual heliostats 120. That is, the heliostats 120 may include processors that substantially independently determine and control the pitch and angle of the heliostats' reflector surfaces.

Although several heliostats 120 are shown in proximity to the receiver 104, there may be more or fewer heliostats and those shown are for illustrative purposes. In the implementation shown, the receiver 104 includes an aperture 110 that is configured to receive solar rays that are reflected from the heliostats 120. In other implementations, the receiver 104 can include a surface that is configured to receive solar rays (i.e., without an aperture) and other configurations of receiver can be used. The heat from the solar rays can be used, for example, to heat a working fluid, e.g., water, air or molten salt. By way of illustrative example, the working fluid can travel through a heat exchanger to heat water, produce steam, and then generate electricity through a turbine connected to a generator. In other examples, the heat is used to heat air or another gas. The heated gas is then expanded through the turbine, which turns a shaft to drive the generator. The electricity can be conducted to a utility grid, or some other point where the electricity can be stored, distributed or consumed. If the solar rays from one or more of the heliostats 120 are directed off-target, or even partially off-target, the heat from those heliostats is lost and the efficiency of the overall system 100 is reduced.

On a front surface 105 of the receiver 104 are four optical signalers 112, 114, 116, 118 positioned proximate to the circumference of the aperture 110. In the implementation shown there are four signalers, however, it should be understood that more or fewer signalers can be used and they can be located in different positions than those shown. When solar rays from a heliostat, for example the heliostat 120 b, are reflected toward the receiver 104 on-target, the reflected solar rays, for example the ray 144, are incident on the aperture 110. However, if a heliostat is off-target, for example the heliostat 120 a, then some or all of the solar rays reflected from the heliostat may miss the aperture 110. For example, the solar ray 140 reflected from the reflective surface 122 of the heliostat 120 a is aiming to a location beneath the aperture 110. Similarly, the heliostat 120 c is off-target and the solar ray 146 reflected from the reflective surface 152 of the heliostat 120 c is aiming to a location to the right of the aperture 110.

FIGS. 2A and 2B show schematic representations of the front surface 105 of the receiver 104 shown in FIG. 1. As discussed above, it should be understood that the target can be a location other than a receiver 104, and the methods and systems described below can be applied in that context. FIG. 2A shows a schematic representation of an image 162 of light incident on the aperture 110 of the receiver from a heliostat that is on-target, e.g., the heliostat 120 b. The center of the image is approximately coincident with the center 160 of the aperture 110. In this example, none of the light reflected from the on-target heliostat is incident on any of the optical signalers 112, 114, 116 and 118. However, it should be understood that generally even when the heliostat is on-target, a small fraction of the reflected light may be incident on one or more of the signalers. For example, imperfections in the reflective surface of the heliostat and/or dust or other contaminants present on the surface can cause some light rays to be reflected off target. A soft edge to the focused light may also cause some light rays to be incident on one or more of the signalers, particularly if they are positioned quite close to the target area. As shall be described further below, a threshold optical signal strength can be set and any optical signals detected by the sensor 138 at the heliostat that are less than the threshold signal strength can be ignored, thereby reducing the likelihood of a false error detection at the heliostat resulting from situations such as those described above. Other factors can be used to determine whether or not to ignore signals received from the signalers, for example, if a weak signal is received from two signalers on opposite sides of the target, then probably the heliostat is on-target (i.e., focused in between the two signalers).

FIG. 2B shows a schematic representation of an image 164 of light incident on the aperture 110 from a heliostat that is off-target, e.g., the heliostat 120 a. The center of the image is beneath the center 160 of the aperture. Some of the light is incident on the aperture 110, however, some is spilling over and the heat from that light is lost. The light that has spilled over beneath the aperture is incident on the signaler 116. For example, the solar ray 140 (see FIG. 1) reflected from the heliostat 120 a misses the aperture 110 but will be incident on the signaler 116 that is positioned beneath the aperture 110. In response to the signaler 116 receiving the solar ray 140, an optical signal 142 is sent from the signaler 116 to the heliostat 120 a that directed the light to the signaler.

The optical signal 142 is detected by a sensor 138 at the heliostat 120 a. In the implementation shown, the sensor 138 is positioned at approximately the center of the reflective surface 122, which can provide optimized cross-talk immunity from the heliostat's neighbors. In other implementations, the sensor 138 can be located at a different position, for example, off-center. In some implementations, a small amount of the reflective surface can be made non-reflective, e.g., as a transparent region, and the sensor 138 can be located behind the transparent region of the reflective surface 122, e.g., on the back side of the mirror. The sensor can thereby be better protected from the environment, yet be positioned in an optimal location. In a particular example, the transparent region can be approximately 0.25 centimeters by 0.25 centimeters in dimension.

In some implementations, a controller 136 is coupled to the heliostat and is configured to determine which particular signaler from the multiple signalers at the receiver (i.e., signalers 112, 114, 116, 118) transmitted the optical signal 142. Optical signals emitted from each of the signalers can be different and the controller can determine which signaler sent the optical signal based on known differences between the signals emitted by each signaler. Based on the optical signal 142, the controller can determine an error in the orientation of the reflective surface 122 of the heliostat 120 a. That is, once the controller has determined that the optical signal came from the signaler 116 that is located beneath the aperture 110, the controller can determine that the reflective surface is reflecting the solar rays too low. In response, the controller 136 can provide a signal to one or more drive systems to adjust the orientation of the reflective surface, for example, to adjust the elevation (pitch) and/or azimuth of the reflective surface.

The example heliostat 120 a shown includes a mirror having the reflective surface 122, which mirror is mounted on a base member 124. The base member 124 is secured to the terrestrial surface 108. In the example shown, the base member 124 is secured to the terrestrial surface by a concrete pad 132. A schematic representation of an azimuthal drive system 130 is shown that is configured to adjust the azimuth position of the reflective surface 122 by rotating the mirror about a vertical axis in the directions of the arrow 125. Although the azimuthal drive system 130 is shown at the bottom of the base member 124, in other implementations it may be closer to the mirror or otherwise positioned. A schematic representation of an elevational drive system 126 is shown that is configured to adjust the elevation position of the reflective surface 122 by rotating the mirror about an elevational axis. The elevational axis is a horizontal axis that is perpendicular to the azimuthal axis and that is directed into the page in this particular drawing. The elevational drive system 126 is configured to rotate the mirror about the elevational axis in the directions shown by the arrow 128.

The controller 136 can be coupled to the azimuthal drive system 130 and the elevational drive system 126 and configured to provide signals to the drive systems 130, 126 to adjust the orientation of the reflective surface 122 to correct for an error determined based on the detected optical signal. The controller 136 can be located at the heliostat 120 a as shown, or can be located remote from the heliostat 120 a, but in communication with the azimuthal and elevational drive systems 130, 126, e.g, by wired or wireless communication.

Because the optical signal is directed back to the heliostat, e.g., heliostat 120 a, the optical signal is not incident on the reflective surfaces or sensors of other heliostats included in the field of heliostats. Therefore other heliostats that are on-target, e.g., heliostat 120 b, do not receive the optical signal. In implementations where the optical signaler is a retroreflector, there can be a relatively small angle error of the returned light. Preferably, the angle error is large enough so that a sensor on the heliostat can detect reflections of light originating across some part of the heliostat, but the angle error is small enough that the light detected by the sensor sees none (or almost none) of the light reflected from a retroreflector that has originated from other heliostats.

The optical signals emitted by the multiple signalers 112, 114, 116, 118 can be different so that the controller at a heliostat can determine which of the signalers sent the optical signal. In implementations where the signals are retroreflectors, various techniques can be used to differentiate the signals. In some implementations, the frequency of the light reflected by each retroreflector can be different. For example, the reflected light from the retroreflectors can be optically chopped by placing a rotating partial disk in front of the retroreflector. If each retroreflector can be modulated with a different frequency, e.g., signaler 112 at 5 Hz, signaler 114 at 10 Hz, signaler 116 at 15 Hz and signaler 118 at 20 Hz, then a simple examination of the light intensity hitting the sensor 138 can be used to resolve which retroreflector the light is coming from. In some implementations, the sensor 138 can be a semiconductor IR (infrared) detector diode or a small solar PV (photovoltaic) cell, although other sensors can be used.

In some implementations, the color of the light emitted from a retroreflector can be different than that emitted from another retroreflector, for example, by using different color filters for the retroreflectors. In some implementations the filter is located in proximity to the signaler and in other implementations, the filter can be located in proximity to the sensor. The color of the light incident on the sensor 138 can be determined to resolve which signaler sent the optical signal. By way of example, the optical signal can be visible (as discussed above) or infrared. In some implementations, a two-part rotating disk can be used to filter the optical signal so that, for example, half the time the signal is a first color and the other half the time the signal is a second color. A two-part color detector can then resolve which signaler sent the optical signal, which can be combined with a correlation analysis with an expected signal to yield improved noise rejection. That is, a property of the expected signal, e.g., square wave or a fixed frequency, can be used together with the two-part color of the signal, so that false signal detection due to signal noise can be reduced. In some implementations, the signal to noise ratio can be further improved by employing: (a) a filter over the sensor 138 that attenuates light and/or restricts the bandwidth of light (including infrared) that reaches the sensor; (b) choosing chopper frequencies in a range where detection is facilitated; (c) choosing polarization filter orientations(s) to null out undesired light sources; and/or (d) using multiple sensors, each with different filter/polarization characteristics, and selecting the cleanest signal as received by the multiple sensors. Other techniques to reduce the signal to noise ratio can be used, and the ones described are illustrative examples.

In some implementations, rather than using a different color, the polarization can be switched. That is, half the time the signal can be a first polarization and the other half the time the signal can be a second polarization. In other implementations, the polarization can be same throughout the duration of the signal, but different polarizations can be used for each signaler.

In some implementations, the frequency and/or phasing of the optical signals from each signaler can be different, and by detecting the frequency and phase of the signal, the origin of the signal can be determined. FIG. 3 is a schematic representation of frequency and phase modulated optical signals and an error detection algorithm. In this representation, for illustrative purposes, optical signalers are positioned on the receiver to detect that the reflected solar rays are high, low, to the left or to the right. The optical signal 302 has a 10 Hz frequency and represents an optical signal from the high optical signaler. The optical signal 304 has a 10 Hz frequency that is anti-phased compared to the optical signal 302, and represents the optical signal from the low optical signaler. The optical signal 306 has a 5 Hz frequency and represents an optical signal from the left optical signaler. The optical signal 308 has a 5 Hz frequency that is anti-phased compared to the optical signal 306 and represents the optical signal from the right optical signaler. The optical signal 310 is an example signal measured at an example sensor.

A schematic representation of an error detection algorithm 312 is shown. The measured signal 310 can be determined to be either a 10 Hz or 5 Hz signal. A low pass filter can be used to reject noise in the signal 310. The example error detection determinations for a 10 Hz signal are shown at 314 and for a 5 Hz signal are shown at 316. For example, for a 10 Hz signal, if the signal is +, then the signal came from the high optical signaler, and the solar rays are too high, thus the error “high”. If an error is detected the control system can send a signal to the drive system to adjust the reflective surface, in an effort to correct the detected error.

In some implementations, the phasing of the optical signals from each signaler can be different, and by detecting the phase of the signal, the origin of the signal can be determined. FIG. 4 is a schematic representation of phased optical signals. In this representation, for illustrative purposes, optical signalers are positioned on the receiver to detect that the reflected solar rays are high, low, to the left or to the right. The optical signal 402 has a first phase and represents an optical signal from the high optical signaler. The optical signal 404 has a second phase, different from the first phase, and represents the optical signal from the low optical signaler. The optical signal 406 has a third phase, different from the first and second phases, and represents an optical signal from the left optical signaler. The optical signal 408 has a fourth phase, different from the first-third phases, and represents the optical signal from the right optical signaler. The optical signal 410 is an example signal measured at an example sensor.

A couple of regions in the measured signal 410 are highlighted, i.e., regions 414 and 418, and are compared to the phased optical signals from the optical signalers as emitted at the same time as the regions of the signal were received at the sensor (i.e., as shown by the encircled areas 412 and 416). The region of signal 414 is most likely attributed to an optical signal received from the low optical signaler, because it is approximately aligned with the phased signal 404 at the given time. A corresponding error detection is that the solar rays are being reflected too low relative to the receiver. Similarly, the region of signal 418 is most likely attributed to an optical signal received from the left optical signaler, because it is approximately aligned with the phased signal 406 at the given time. A corresponding error detection is that the solar rays are being reflected too far to the left relative to the receiver.

In the example described above a filter or chopper can be used to change a signal from an optical signaler, so as to differentiate the signal from those emitted from other optical signalers. In some implementations, the filter or chopper is located in proximity to the optical signaler. In other implementations, the filter or chopped is located in proximity to the sensor. In the examples discussed above, there is one sensor at the heliostat, however, it should be understood that in other implementations, there can be two or more sensors per heliostat.

Although a retroreflector does reflect light back toward the source of the initial light beam incident on the retroflector, and therefore back to the heliostat that is aiming off-target, in a heliostat field with closely spaced heliostats, it is possible for some noise to occur in optical signals from the retroreflectors, in addition to noise from random signals present in the environment. In some implementations, a threshold optical signal strength can be set and any optical signals detected by the sensor 138 at the heliostat that are less than the threshold signal strength can be ignored, thereby reducing the likelihood of a false error detection at the heliostat.

It is possible for the sensor 138 to detect an optical signal from more than one optical signaler at the same time. For example, during typical operation of reflecting solar rays toward a target, the image of sunlight from the heliostat may be large enough that there is spill on both the left and right sides of the target, or the mirror may have an imperfection (e.g., dust on the surface) so that some light is visible outside the “perfect” reflection angle. As a result, an optical signaler to the left of the target location and an optical signaler to the right of the target location can both be emitting signals, probably of varying strengths, back to a heliostat. In some implementations, the difference and the relative magnitude of the two optical signals can be used to determine how far off center the heliostat is actually aiming. If the difference is negligible, then the controller may do nothing in response. If the difference is non-negligible, then the amount of reaction by the controller may be proportional to the difference. By way of illustrative example, consider a sensor that can register 0 to 1 volt. An imbalance (i.e., difference) of +/−0.1 volt may be ignored, whereas an imbalance greater than +/−0.1 may result in the controller commanding the drive system to make a corrective adjustment. In some implementations, the rate of the corrective movement may be proportional to the imbalance. That is, an imbalance of 0.2 volt can result in a slow rate of motion for correction, while an imbalance of 0.8 volt may cause a larger rate of motion for correction.

In the implementation shown, the signalers are positioned about the circumference of the aperture 110 at approximately every 90 degrees. In other implementations, the signalers can be positioned further or closer to the aperture, and more or fewer signalers can be used. For example, eight signalers can be used that are positioned at approximately every 45 degrees. In another example, eight signalers can be used that are positioned in pairs at approximately every 90 degrees, with one signaler in each pair located a further distance from the center 160 of the aperture. In some implementations, at least one signaler can be positioned on a movable member, e.g., a pole, that swings out and back toward the aperture 110. The signaler can spend more time at one position, e.g., the position further away from the aperture (i.e., when the pole is swung out), so that a longer reflective pulse is emitted the further off target the heliostat. Having signalers that are positioned at both different positions and distances relative to the aperture provides information to the heliostat about not only the direction of the misalignment but how far off target the reflected light is when incident at the receiver.

In some implementations, the desired target of the solar rays reflected from one or more heliostats may not be the aperture of the receiver, but rather a position away from the receiver. For example, during high solar intensity periods of the day, it may be desirable to aim one or more heliostats away from the receiver, to avoid damage to the receiver from excessive temperatures. One or more optical signalers can be positioned accordingly, so as to be used during those times where the desired target of one or more heliostats is somewhere other than the aperture.

FIG. 5 shows a block diagram representing an example system 500 that includes a receiver and a heliostat. For illustrative purposes, the heliostat is heliostat 120 a and the receiver is receiver 104 as shown in FIG. 1. The receiver 104 includes multiple signalers, and in this particular example, the four signalers that are also depicted in FIG. 1, being the top signaler 112, the bottom signaler 116, the left signaler 118 and the right signaler 114. The heliostat 120 a includes the reflective surface 122 and a drive system 131. The drive system 131 includes the azimuthal drive system 130 and the elevational drive system 126. The heliostat 120 a further includes a controller 136. The controller includes a sensor 138, a demodulator 170 and a resolver 172. The controller is configured to provide signals to the drive system 131 to adjust the azimuth and elevation positions of the reflective surface 122.

The sensor 138 included in the controller can be, as mentioned above, a semiconductor IR detector diode or a solar PV cell, although other types of sensor can be used. The demodulator 170 is configured to determine a property of an optical signal detected by the sensor 138, such that a determination can be made as to which signaler sent the optical signal. For example, the demodulator 170 can be most responsive to the frequency, color, polarization or phase of the optical signal. Example demodulators and techniques include, but are not limited to, correlation decoders, four quadrant multipliers, low pass filtering (to reject noise), and/or use of differential signals to create a center-null.

The resolver 172 is configured to determine which signaler transmitted the optical signal based on an output from the demodulator 170. For example, if the demodulator outputs that the frequency of the optical signal is 5 Hz, which is the known frequency for the bottom signaler 116, then the resolver 172 can resolve that the optical signal came from the bottom signaler 116 (e.g., as shown in FIG. 2B). The resolver 172 can further determine that the heliostat's reflective surface 122 is aiming the reflected sunlight too low relative to the aperture of the receiver and determine a corrective action to resolve the error. In this example, the resolver 172 can instruct the elevational drive system 126 to adjust the elevation (i.e., the pitch) of the reflective surface 122 upward, so as to direct the solar rays higher and on-target. In some implementations, the resolver 172 can be implemented as an analog circuit. In other implementations, the resolver 172 can be implemented as a digital system with a processor executing software. The software can be executing on a per-heliostat processor or on a processor that controls a set of multiple heliostats. Other configurations of resolver 172 are possible.

FIG. 6 is a flowchart showing an example process 600 for maintaining a heliostat on-target. In the example described, the target is a receiver, however, as discussed above, the target can be a location other than a receiver and the process 600 can be applied in such instances as well. Solar rays incident on a reflective surface of the heliostat are reflected toward a receiver (Box 602). By way of illustrative, the process 600 can be described in reference to FIG. 1, although it should be understood that differently configured receivers and heliostats can be used to carry out the process 600 than those depicted. In this example, the heliostat 120 a reflects solar rays, e.g., ray 140, that are incident on its reflective surface 122 toward the aperture 110 of a receiver 104.

As was discussed above, in an ideal operating situation, when the heliostat is on-target, all of the reflected solar rays are reflected to the target and none of them are incident on signalers positioned about the target. However, the more typical operating situation is that at least a small fraction of the solar rays will always be reflected off-target, e.g., due to dust on the reflective surface, minor imperfections in the reflective surface or otherwise. As such, at least one of the signalers will always be sending an optical signal, albeit of a relatively weak strength, back to the on-target heliostat. The process 600 is directed to such circumstances.

A determination is made whether an optical signal is detected from a signaler at the receiver (604). If a determination is made that an optical signal is detected (“Yes” branch of 604), then a determination is made whether a threshold signal strength is exceed by the detected optical signal (606). That is, as discussed above, a threshold signal strength can be set to weed out weak optical signals that may be generated from a small fraction of solar rays that are directed off target from an otherwise on-target heliostat. If the threshold signal strength is not exceeded (“No” branch of 606), then the detected optical signal is ignored (Box 608), and the process 600 can loop back to 604.

If the threshold signal strength is exceeded (“Yes” branch of 606), then a determination is made as to which optical signaler sent the detected optical signal (Box 610). For example, the sensor 138 can detect an optical signal, e.g., signal 142, that was sent from the bottom signaler 116 at the receiver 104. The demodulator 170 included in the controller 136 can determine a property of the optical signal 142 and the resolver 172 included in the controller 136 can determine, based on the property, which signaler sent the signal.

A strategy to adjust the position of the reflective surface can be determined based the determination as to which signaler sent the signal (Box 612). For example, the resolver can determine that because the bottom signaler 116 sent the signal, the solar rays are being reflected too low and determine a strategy to adjust the orientation of the reflective surface 122 in elevation so as to raise the solar rays reflected toward the receiver 104.

The orientation of the reflective surface is adjusted based on the determined adjustment strategy (Box 612). For example, the controller 136 can send a signal to the elevational drive system 126 to adjust the elevation of the reflective surface 122 so that the solar rays are reflected to a higher position relative to the aperture of the receiver 104.

The process 600 then loops back to 604. If there is still an error in the orientation, then an optical signal that exceeds the threshold strength will still be detected at 604 and 606, and the reflective surface adjusted accordingly (i.e., at 612).

Referring back to step 604, if no optical signal is detected (“No” branch of 604), then it is possible the heliostat is grossly off-target, a cloud event has obscured the Sun to the extent that solar reflection from the receiver is grossly diminished or the heliostat is perfectly on-target and no solar rays are inadvertently being reflected off-target. As discussed above, this example process 600 assume that the third possibility is not an option, although in some system configurations it could be (e.g., signalers are positioned a significant distance away from the target). For purposes of this process 600 however, the third possibility is not treated as an option. A determination is made whether cloud cover is detected (614). For example, one or more solar intensity sensors can be included in the system, and based on data from the sensors, a determination can be made that based on the solar intensity, cloud cover is likely obscuring solar rays from the Sun. If a determination is made that cloud cover likely has caused the loss of optical signal acquisition (“Yes” branch of 614), then the process loops back to step 604 and assumes that the heliostat has remained on-target.

If a determination is made that cloud cover is not detected (“No” branch of 614), then the heliostat is presumed to be grossly off-target, which has resulted in the loss of signal acquisition. The orientation of the reflective surface is adjusted until an optical signal is detected (Box 616). In some implementations, the orientation can be adjusted to put the heliostat at least coarsely on-target, at which point signal acquisition resumes, and following which the process can loop back to step 604 for fine-tuning adjustment to put the heliostat on-target. The coarse adjustment can be based on the difference between ideal heliostat orientation and an estimated heliostat orientation using data from one or more sensors, for example, data from an inclination meter, accelerometer and/or a magnetic compass and potentially combined with dead-reckoning from a pre-calibrated or known orientation, although other techniques can be used.

In some implementations, by default, the controller can play catch-up with the continual motion of the reflective surface due to the motion of the Sun of the sky. If the controller and the drive system of the heliostat act slowly in response to errors, the image of sunlight cast on the receiver (or other target) may lag by an unacceptable amount. To rectify this condition, the drive system acting in concert with the controller can be commanded to move the reflective surface at a rate so as to compensate (at least approximately) for the known motion of the Sun. By using such feed-forward rate commands, the image of sunlight projected by the heliostat on the receiver will remain closer to on-target.

The above example process 600 assumed that if signal acquisition ceased altogether, it was the result of either a cloud event (or other environmental occurrence) or gross mis-alignment of the heliostat. However, in some configurations, e.g., where the signalers are positioned far enough away from the target to avoid inadvertent stray solar rays from an otherwise on-target heliostat, is signal acquisition ceases altogether, the heliostat is assumed to be right on-target and no adjustments are made until an optical signal is detected.

A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention.

In addition, the logic flows depicted in the figures, for example process 400 in FIG. 4, do not require the particular order shown, or sequential order, to achieve desirable results. In addition, other steps may be provided, or steps may be eliminated, from the described flows, and other components may be added to, or removed from, the described systems. Accordingly, other embodiments are within the scope of the following claims. 

1. A method for managing heliostat aiming toward a target, comprising: reflecting solar rays incident on a reflective surface of a heliostat mirror toward a target, wherein one or more optical signalers are arranged at positions about the target; detecting an optical signal received from one of the one or more optical signalers; and determining an error in an orientation of the reflective surface based on the optical signal.
 2. The method of claim 1, further comprising: adjusting the orientation of the reflective surface in response to determining the error.
 3. The method of claim 2, wherein adjusting the orientation comprises adjusting at least one of the azimuth or elevation of the reflective surface.
 4. The method of claim 2, wherein adjusting the orientation comprises adjusting the orientation along an axis other than along an azimuthal or elevational axis of the reflective surface.
 5. The method of claim 1, wherein one or more optical signalers comprise a plurality of optical signalers and determining an error in an orientation includes determining which optical signaler of the plurality of optical signalers transmitted the optical signal.
 6. The method of claim 1, wherein the one or more optical signalers comprise a plurality of retroreflectors positioned about the target.
 7. The method of claim 6, wherein: the optical signal is received from a particular one of the plurality of retroreflectors; and determining the error in the orientation comprises: determining a frequency of a change in light that comprises the optical signal; and based on the frequency, determining that the optical signal was received from the particular one of the plurality of retroreflectors; and based on determining that the optical signal was received from the particular retroreflector, determining the error in orientation.
 8. The method of claim 6, wherein: the optical signal is received from a particular one of the plurality of retroreflectors; and determining the error in the orientation comprises: determining a color of light that comprises the optical signal; and based on the color, determining that the optical signal was received from the particular one of the plurality of retroreflectors; and based on determining that the optical signal was received from the particular retroreflector, determining the error in orientation.
 9. The method of claim 6, wherein: the optical signal is received from a particular one of the plurality of retroreflectors; and determining the error in the orientation comprises: determining a polarization of light that comprises the optical signal; and based on the polarization, determining that the optical signal was received from the particular one of the plurality of retroreflectors; and based on determining that the optical signal was received from the particular retroreflector, determining the error in orientation.
 10. The method of claim 6, wherein: the optical signal is received from a particular one of the plurality of retroreflectors; and determining the error in the orientation comprises: determining a phasing of light that comprises the optical signal; and based on the phasing, determining that the optical signal was received from the particular one of the plurality of retroreflectors; and based on the determining that the optical signal was received from the particular retroreflector, determining the error in orientation.
 11. The method of claim 1, wherein determining an error in an orientation of the reflective surface based on the optical signal includes determining that the optical signal exceeds a threshold signal strength.
 12. The method of claim 1, wherein two optical signals are received from two optical signalers and determining an error in an orientation of the reflective surface based on the two optical signals includes determining that a difference in signal strength between the two optical signals exceeds a threshold difference.
 13. The method of claim 1, wherein the target comprises a receiver configured to receive solar rays.
 14. The method of claim 1, wherein the target comprises a location that is a distance away from a receiver that is configured to receive solar rays, such that solar rays from the heliostat mirror are not reflected to the receiver.
 15. A system comprising: a receiver assembly including: a receiver mounted on a receiver tower; the receiver tower comprising a support structure configured to support the receiver; an aperture included in the receiver and configured to receive solar rays reflected from a plurality of heliostats; one or more signalers positioned at one or more distances from the aperture and configured to: receive solar rays reflected from at least one of the plurality of heliostats; and in response to transmit an optical signal toward the at least one heliostat.
 16. The system of claim 15, wherein the one or more signalers comprise one or more retroreflectors mounted at one or more positions proximate to a circumference of the aperture.
 17. The system of claim 15, wherein the one or more signalers comprise a plurality of signalers and each signaler is configured to transmit an optical signal that is different than an optical signal transmitted by each of the other signalers in the plurality of signalers.
 18. The system of claim 17, wherein each signaler comprises a retroreflector that is configured to transmit an optical signal that has a different frequency of change in signal than optical signals transmitted by the other signalers in the plurality of signalers.
 19. The system of claim 17, wherein each signaler comprises a retroreflector that is configured to transmit an optical signal that has a different color than optical signals transmitted by the other signalers in the plurality of signalers.
 20. The system of claim 17, wherein each signaler comprises a retroreflector that is configured to transmit an optical signal that has a different polarization than optical signals transmitted by the other signalers in the plurality of signalers.
 21. The system of claim 17, wherein each signaler comprises a retroreflector that is configured to transmit an optical signal that has a different phasing than optical signals transmitted by the other signalers in the plurality of signalers.
 22. The system of claim 15, further comprising: the plurality of heliostats, wherein each heliostat includes: a reflective surface configured to reflect solar rays incident on the surface toward the receiver; a sensor configured to receive optical signals from the one or more signalers; an actuator configured to adjust an orientation of the reflective surface; a controller configured to: determine errors in orientation of the reflective surface based on the optical signals received by the sensor; and provide signals to the actuator to adjust the orientation of the reflective surface in response to the determined errors.
 23. The system of claim 22, wherein: the one or more signalers comprise a plurality of signalers, and each signaler is configured to transmit an optical signal that is different than an optical signal transmitted by the other signalers in the plurality of signalers; and the controller is further configured to determine which particular signaler from the plurality of signalers transmitted a particular optical signal based on the difference in optical signals transmitted by the plurality of signalers.
 24. The system of claim 22, wherein the controller configured to determine errors in orientation of the reflective surface is further configured to determine that the optical signals exceed a threshold signal strength.
 25. The system of claim 22, wherein the controller configured to determine errors in orientation of the reflective surface is further configured to determine whether a difference between signal strengths of two optical signals received from two optical signalers exceeds a threshold difference and determine an error in orientation based on the determination. 